Generally the aqueous phase embedded in the precipitates of cellulose acetate prevailingly comprises acetic acid (ca. 30%) water (ca. 70%) and small quantities of sulphuric acid.
It is well known that cellulose acetate is produced by esterification of cellulose with acetic anhydride in glacial acetic acid as a solvent and using sulphuric acid as catalyst. Diluted acid is used to hydrolize the ester and the sulphate groups. Cellulose acetate is then precipitated by further addition of the diluted acetic acid.
As an example the cellulose acetate flakes are formed of about 15% of cellulose acetate and about 85% of a solution comprising acetic acid, water, sodium sulphate and small amounts of sulphuric acid. The flakes are usually washed with water a pH of 7 is obtained in the washing liquid.
The washed flakes contain ca. 85% by weight of aqueous solution. The flakes are then pressed to reduce the water content to ca. 50% by weight and then dried to a water content of about 1% by weight (Encyclopaedia of Polymer Science and Engineering, 2nd Edition, Vol. 3, page 158, Wiley-Interscience Publication, 1985). This separation process produces remarkable amounts of waste water which need purification before being emitted into the surrounding.
The prior process also produces a great amount of diluted acetic acid which has to be recovered to render the process economically practicable.
The recovery of acetic acid is based on the azeotropic distillation with organic solvents to separate the acid from water.
It has been found that it is possible to reduce the content of the aqueous phase (mainly water and acetic acid) present in the flakes of cellulose acetate precipitated from the solution of esterification of cellulose (the flakes contain up to about 85% or more of a solution comprising water, acetic acid and small amounts of sulphuric acid) without using the numerous washing steps with water and the subsequent drying step which are representative of the processes of the prior art.